Neurons communicate with each other by means of chemical transmitter substances. The physiological stimulus for transmitter release appears to be a depolarization-induced influx of calcium into the axon terminal, but the presynpatic processes intervening between calcium influx and transmitter release remain unknown. The proposed study will focus on those factors which determine the time course with which trasmitter is released from depolarized presynpatic axon terminals. The time course of evoked transmitter release will be measured by computer-assisted analysis of end-plate potentials recorded intracellularly or (focal) extracellularly at the neuromuscular junction. The objectives of the proposed research are (1) to determine (a) what factors influence the time course of evoked transmitter release and (b) whether the early and late components of evoked release have similar or dissimilar mechanisms, by testing the kinetic effects of variables known to alter the magnitude of evoked release (including (Ca), repetitive nerve stimulation, temperature, ionic strengths and inhibitors of mitochondrial Ca pumps); (2) to measure and analyze the relationship between the late component(s) of evoked release and the facilitation of release evoked by a second depolarization, in the light of current models of transmitter release, and to determine whether facilitation is graded with external (Ca), and (3) to determine the time course and ionic dependencies of a recently described interaction between spontaneous and evoked release. The techniques outlined here will allow both short- and long-term phenomena of transmitter release to be studied in the same preparation, thereby allowing quantitative tests of various proposed transmitter release mechanisms.